The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 0.25 inch up to about 1.0 inch, and height ranging from less than 0.15 inch up to about 0.60 inch. The small size and the limited amount of electrochemically reactive material which can be contained in these small metal-air cells result in considerable attention being directed to improving the efficiency and completeness of the power generating electrochemical reactions which occur therein.
Metal-air cells convert atmospheric oxygen to hydroxyl ions in the air cathode. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc.
More particularly, the desired reaction in a metal-air cell air cathode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions, the hydroxyl ions being able to migrate through the electrolyte toward the anode, where oxidation of zinc may occur, forming zinc oxide.
In most metal-air cells, air enters the cell through one or more ports extending through the bottom of the cathode can. The ports may be immediately adjacent the cathode assembly, or may be separated from the cathode assembly by an air chamber or an air diffusion member.
In any of such arrangements, the port: facilitates the movement of air from the outside environment into the cathode can, and thence into the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms hydroxyl ions.
In normal operation, the reaction surface of the cathode assembly is laden with electrolyte, water typically being a major constituent of the electrolyte. Accordingly, the water at the reaction surface of the cathode assembly has a vapor pressure, and is subject to evaporation at the reaction surface. Moisture evaporated from the reaction surface of the cathode assembly can escape from the cell through the port, whereby the cell dries out if the relative humidity is below the equilibrium humidity, and correspondingly loses effectiveness. Thus, there is a relationship between the amount of oxygen that can be made available to the cell through conventional port configurations, and the amount of moisture loss associated with such port configurations.
It is an object of this invention to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having one or more air entry ports so structured and configured, both individually and relative to each other, that the port configuration provides an improved relationship between the amount of oxygen that is available to the cathode assembly and the amount of moisture lost from the cell through the port configuration.
It is another object to provide improved cathode can structure for a metal-air electrochemical cell, wherein the sum of the open area of the port configuration is reduced while maintaining the cell limiting current.
It is still another object to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having a plurality of ports, with the port configuration structured so that, in a metal-air cell made with the cathode can, oxygen is more uniformly distributed over the cathode assembly, while minimizing the combined open area of the ports through the cathode can, and thereby reducing the amount of moisture transmitted into, or out of, the cell through the ports.
A further object is to provide improved metal-air electrochemical cells having an increase in the ratio of the limiting current of the cell to the combined area of gaseous ingress and egress available through the port configuration when the cell is in use.
Yet another object is to provide improved cathode can structure having ports with stepped port configurations, including a first larger port opening for facilitating air entry through a covering tab prior to the cell being put into use, and a second smaller port opening for limiting the air entrance and moisture loss while the cell is in use.
Still another object is to provide improved metal-air cells using the reduced port area for controlling ingress and egress of air when the cell is untabbed and in use, a covering tab having a permeability to air insufficient to maintain, over the reduced port area, an open cell voltage within the operating voltage range, the cell including ports incorporating stepped openings, thus presenting a tabbed diffusion area larger than the reduced port area.